This invention relates to a method and apparatus for monitoring the recording output of a laser driven, computer output microfilmer.
A "computer output microfilmer" (hereafter COM) is a system in which output data from a computer is optically superimposed on a slide image provided with format lines, and recorded on a microfilm. In conventional COM's the output data is displayed in the form of numerals, characters or the like on a Braun tube, and the optical image thereof is focused onto a silver salt film through an appropriate lens system.
Recently, a number of highly stable and inexpensive laser systems have been developed, and this has led to the use of laser optical sources in COM's. The laser beam from such a source is intensity-modulated and two-dimensionally deflected by an optical scanning device to sweep over and record on a microfilm. The laser COM has much higher brightness and energy characteristics than a COM employing a Braun tube, whereby relatively cheap and insensitive recording materials may be used, such as a Dry Silver (TM) film, a diazo film, or a thin metal heat-mode film. Thus, the laser COM offers the advantage of avoiding a costly and troublesome wet film development process.
With a COM it is impossible to observe the computer printout in situ; that is, the exposed microfilm must first be developed and then enlarged by a reader or projector. Accordingly, it is necessary to monitor the optical registration between the Braun tube and format slide images, the focusing conditions, etc., in order to ensure satisfactory recording. To meet this requirement conventional COM's are provided with optical viewing monitors operable through a half-mirror or a semi-transparent mirror. In monitoring a Braun tube it is necessary to repeatedly display the image, which may cause a tiring flickering phenomenon depending upon the value of the repetition frequency.
In existing COM's the microfilm recording rate varies from about one frame per second to more than ten frames per second, and the image repetition frequency correspondingly varies from about 1 Hz to about 10 Hz. When an image formed at a low repetition frequency is monitored a slight flickering phenomenon may be observed, but this is not particularly bothersome in a Braun tube because its phosphor has a relatively slow decay rate. In a laser COM, however, flickering is very pronounced at repetition frequencies on the order of several Hz because there is no phospor decay, and as a result the observers eyes tire easily.
The flickering phenomenon depends on the contrast of the black and white image pattern, and tends to decrease as the repetition frequency is increased. The acceptable repetition frequency, called the critical fusion frequency, is thus a function of image brightness, and generally lies in the range from 10-20 Hz. With a laser COM it is possible to monitor the image by suitably controlling the brightness thereof, and the frequency at which the flickering phenomenon is reduced to the point where the observers eyes do not become tired is generally higher than 7 Hz, and preferably higher than 10 Hz.